The present invention relates to a novel method and apparatus for accurately measuring and indicating the thrust developed by turbofan jet engines of a type for which conventional EPR computing indicators do not develop pressure ratio measurements that are consistently and reliably correlated to the level of thrust. Namely, the type of engine with which the invention is concerned is low exhaust noise turbofans wherein the turbine and fan discharges are combined in a mixer being discharged from the engine nozzle.
With conventional turbojets and nonmixer turbofans, existing EPR measuring indicators yield results that consistently and accurately indicate the level of developed thrust, from engine to engine of a production run (after calibration on a test engine of the same type and model). Such indicators take the ratio between inlet air pressure and the exhaust gas pressure and the result is used to determine the net thrust of the engine from prepared tables.
However, it is found that the same pressure ratio is not consistently correlated to engine net thrust with mixer-type nozzle turbofans. This is due to manufacturing variations (tolerated to minimize the production cost) in mixer dimensions from engine to engine that substantially affect the relationship or correlation between such pressure ratio and net thrust.
The consequent potential for a range of error in the indicated thrust meant that jet aircraft with such engines would have had to be underrated on certificaton, denying them access to certain airports and the license to carry loads as heavy as they are actually powered to carry. Accordingly, an object hereof is to provide consistently accurate thrust determining measurement apparatus for such turbofan jet engines, and more particularly to provide improvements that may be readily and inexpensively adapted to existing aircraft that would have heretofore been underrated. More specifically, it is an object to provide a means for so deriving and combining engine gas pressures that the resultant may be applied directly to operate existing pressure sensitive electrical computing indicator devices already furnished with existing aircraft, and of making the adaption with minimal cost and with consistently accurate results.